1. Field of the Invention
This invention relates generally to a solid state color television camera, and more particularly to a luminance circuit in a camera which employs a pair of solid state imaging devices.
2. Description of the Prior Art
In the prior art, a television camera has been proposed which employs a semiconductor element, for example, a CCD (charge-coupled device) as an image pick-up device. It also has been known to provide a solid state color camera in which two CCD chips are used as image pick-up devices, with one CCD chip being used to develope a first color signal such as a green color signal, and the other CCD chip being used to develop second and third color signals such as red and blue color signals, line-by-line. For such structures, as shown in FIG. 1, a green color filter 2G is mounted in front of one image pick-up device 1G, and a laterally striped color filter 2RB is mounted in front of the other image pick-up device 1RB so as to produce red and blue color signals in a line-sequence manner.
With an interlace scanning system, the arrangement or alignment pitches Pg of the picture elements and the color striped filter 2RB are selected so that two picture elements are included in one pitch of the filter 2RB in the vertical direction.
An image of an object may be projected on one image pick-up device 1RB, so as to be shifted by 1/2 Px (where Px represents the alignment pitch of picture elements in the horizontal direction) in the lateral direction from an object image, which may be projected on the other image pick-up device 1G. In FIG. 1, the image pick-up device 1RB is shifted by 1/2 Px from the image pick-up device 1G so as to illustrate the above conditions.
The above described image pick-up system is advantageous for synthesizing a luminance signal. Since the sampling by the image pick-up or imaging device are opposite in phase, the carrier and associated side band components, which will cause distortion when synthesizing the luminance signal, can be cancelled.
FIG. 2 illustrates a prior art circuit 10 which is used to synthesize a luminance signal, especially a luminance signal Y.sub.WH of a high frequency band. In the luminance signal forming or synthesizing circuit 10 of FIG. 2, a terminal 3G is supplied with a green signal G provided by the image pick-up device 1G and a terminal 3RB is supplied with line-sequence red and blue signals R and B provided by the image pick-up device 1RB, respectively. In order to place the red and blue signals R and B in synchronism, a delay line circuit 4 of one horizontal scanning period (1H) is provided as shown. The delayed red and blue signals and the non-delayed red and blue signals and the green signal are applied to an adder 5 to produce a luminance signal Y.sub.W having a wide frequency band. This luminance signal Y.sub.W is fed to a band pass filter 7 where it is limited to a frequency band. In this example, a luminance signal Y.sub.WH of high frequency band such as 0.7 to 4.5 MH.sub.z is obtained from the band pass filter 7. The low frequency band signal of the luminance signal Y.sub.W is provided by a separate circuit so as to satisfy the relationship of the luminance signal as specified in the NTSC system. In this example, the adder 5 and delay line circuit 4 of 1H form a comb filter 6A.
Let it be assumed that a blue signal B.sub.n+1 is obtained from the (n+1)' line. When the red and blue signals R and B are attenuated by 1/2 by passing through variable resistors 8A and 8B respectively, a luminance signal Y.sub.Wn+1 on the (n+1)' line can be expressed as follows: ##EQU1##
Since the image pick-up devices 1G and 1RB are spatially shifted by 1/2 Px with respect to the projected image, the sampling carrier of the green signal G will be opposite in phase to that of the red and blue signals R and B. Therefore, if an object to be picked up has the input level of G=1/2(R+B) such as a black and white image, the side band components of the green signal G and the red and blue signals R and B will cancel each other, and hence no side band components remain in the base band (especially in the high band components) of the luminance signal YW. This allows the aliasing or folded distortion can be removed.
Further, even if an object to be picked up is not black and white, the noise on a reproduced picture caused by the aliasing error can be substantially suppressed.
In case of utilizing the circuit 10 shown in FIG. 2, the side band component is somewhat cancelled with an object includes a pattern which will change in the vertical direction, but none of the side band components are cancelled when the object includes a pattern which changes in a slant direction and hence an aliasing distortion will be generated. This is caused by the difference between the transfer characteristic for the green signal G (refer to a curve L1 in the graph of FIG. 3A) and the transfer characteristics for the red and blue signals R and B (refer to a curve L2 in the graph of FIG. 3A).